As the use of broadband in digital mobile radio systems spreads, even higher efficiency in a power amplifier is desired.
Here, referring to FIG. 1, the input/output characteristic and the efficiency of an amplifier is examined. In FIG. 1, the horizontal axis is input power (W), the left vertical axis is output power (W), and the right vertical axis is efficiency. The input power versus the output power characteristic (input/output characteristic) I shows a characteristic that an output growth saturates when the input power (W) becomes a large level. Meanwhile, in regard to the efficiency, the efficiency is low in a region of a small input level, but becomes abruptly improved when the input power reaches a certain value or higher. In short, it is known as a general efficiency characteristic II that the efficiency becomes the maximum when the input power has a value such that the output growth saturates.
Also, when the drain voltage of the power amplifier is varied, the input/output characteristic also varies, as shown by the both-direction arrow in FIG. 1, and the magnitude of the input power producing saturation in the output growth varies as well.
FIG. 2 shows a diagram illustrating a signal waveform after FM analog modulation is performed. When amplifying such the FM analog modulated signal, since the magnitude of the input power is constant, if the drain voltage of the power amplifier is set to such a level that the output power is clipped, the amplifier can be operated at high efficiency, from the efficiency characteristic shown in FIG. 1. However, since the frequency spectrum band of the FM analog modulated signal is broad, it is not advantageous in view of frequency use efficiency.
In particular, with the increase of the number of telephones under contract in a mobile radio system, more efficient use of frequency bands is essential. From this point, a linear modulation scheme having high frequency use efficiency, such as Quadrature Phase Shift Keying (QPSK), has come into use in recent years.
FIG. 3 shows a diagram illustrating a modulated signal waveform according to the linear modulation scheme such as QPSK. In this case, in the region (A in FIG. 3) having a small-level modulated signal, even if a predetermined drain voltage is set, amplification is performed in the linear region of the input/output characteristic I, which is a region of small efficiency. Therefore, it is difficult to increase the total efficiency of the power amplifier.
Here, as the conventional technique to improve power amplifier efficiency, use of switching mode amplifier has been proposed (Patent document 1). The switching mode amplifier described in the above Patent document 1 has a configuration to perform switching mode operation in the own amplifier, and it is also mentioned about an ultra high-speed clock frequency for controlling the switch, such as 256 GHz. However, when a steep pulse of 256 GHz, etc. is used, because matching cannot be made, it is difficult to put into practical use.
[Patent document 1] The official gazette of the Japanese Unexamined Patent Publication No. 2003-92522.